From Brave Nine Wiki
Stat Description
ATK The numerical value of a unit's damage on another unit (Basic ATK max bonus: +300% and max reduction: -80%).
Support The strength of Supporter's buffs. The higher this value, the more powerful the unit's buffs. For example, an effect that increases ATK by 20% at 100% Support increases ATK by 30% at 150% Support.
HP The numerical value of a unit's ability to withstand incoming damage. When this drops down to 0, unit retreats from battle.
DEF % of incoming DMG being decreased. If the Mercenary's DEF is 30% then it will only get 70 DMG from 100 incoming DMG.
Crit Rate Probability of Crit Hit when attacking.
Crit DMG The strength of a unit's Critical damage. 100% Crit DMG doubles the unit's Critical damage.
AGI Affects a unit's chance of being Grazed by an enemy. Grazed attacks inflict 35% less damage, and reduces enemy's Debuff duration time by 50%.